Topology and its Applications 36 (1990) 253-264 253 North-Holland ANOTHER DOWKER PRODUCT Amer BESLAGIe Department of Mathematics, George Mason University, Fairfax, VA 22030, USA Received 4 November 1988 The continuum hypothesis implies that there are (normal) countably paracompact spaces X and Y such that the product X x Y is normal and not countably paracompact. AMS (MOS) Subj. Class.: 54B10, 54D15, 54620, 03E50 product normal countably paracompact Lusin set 1. Introduction A normal space which is not countably paracompact is called a Dowker space, after an old result by Dowker that a normal space X is countably paracompact iff the product X x I is normal, where I is the closed unit interval [5]. To appreciate difficulties in making a space normal but not countably paracom- pact, recall that normality is equivalent to the statement: For every jinite closed family {F,, : n < k} (k < o) with n,,<k F., = 0 there is an open family (0, : n < k} with nnck 0, = 0 and F,, c 0, for each n. And a space is normal and countably paracompact iff for every countable closed family {F, : n < k} (k s w) with nnck F,, = 0 there is an open family { 0, : n < k} with n,,<k 0, = 0 and F, = 0, for each n. There is essentially only one known Dowker space in ZFC, constructed by Rudin [lo]; all its cardinal functions are large. Watson [15] uses a supercompact cardinal to construct another (large) example. If we go beyond ZFC, there are several “small” Dowker spaces: de Caux [3] uses 0, and Rudin [9, 111 a Suslin line to get them. In [6] Rudin uses the continuum hypothesis (CH) to construct another small Dowker space, and her construction is modified by Weiss [ 161, who uses p = c + O,(E), and Bell [ 11, who uses p = c only, to construct two more examples. For more on Dowker spaces see [12]. In [ 131, Rudin and Starbird show that if X is countably paracompact, M metric, and X x M normal, then X x M is countably paracompact. In the same paper they ask if the product of two countably paracompact spaces can be Dowker. Wage [ 141 announced that CH implies that the answer is yes, but he never published his 0166-8641/90/$03.50 @ 1990-Elsevier Science Publishers B.V. (North-Holland) 254 A. Beflagid / Another Dowkerproduct example. Assuming 0 there is a countably paracompact space X such that X xX is Dowker [2]. Here we assume CH and construct a countably paracompact space whose square is Dowker. We use an Ostaszewski technique [8] and van Douwen’s idea from [4]. Our space is a modification of Bell’s line [l]. The key idea of using a Lusin set is from [6]. A subset L of the real line is Lusin iff L is uncountable and every nowhere dense subset of L is countable. CH implies that there are Lusin sets in the line. 2. Construction We need some notation. Let A = {(Y< w 1: a is a limit ordinal}, and A’ = {cy < o, : a is a limit of limit ordinals} (i.e., A’ is the set of accumulation points of A). For any two X and Y, II will always denote the projection map from X x Y onto the first factor X. The sets X and Y will be clear from the context. If (X, a) and (Y, T) are topological spaces we let (T x 7 denote the product topology on X x Y. (Formally, u x 7 is a topology only in trivial cases, but it is a basis for the product topology. We hope that this causes no confusion.) A subset A of the plane R x R is called 2-uncountable iff there is no countable D c R so that A c (D x W) u (R x 0). Otherwise we say that A is 2-countable. In [4] 2-uncountable sets are called big sets. 2.1. Theorem. Assume CH. There exist countably paracompact spaces X and Y such that the product X x Y is Dowker. Proof. Let L be a Lusin subset of the real line and let p denote the Euclidean topology on L. Hence p x p denotes the Euclidean topology on Lx L. Also, let A={(x,y)~LxL:x=y}. Let (x a : a < wJ be a l-l enumeration of L for which there is a pairwise disjoint family { Si : n E o A A E [L x L]“} u { TA : A E [L x L]“} of stationary subsets of w, so that: (a) (VAE[LXL]“)(V~<~,)[~ET,-+~+~ET,], (b) (VA E [Lx L]“)[(clPx,A)\A is 2- uncountable+ {(x,, x,,,): (YE TA} c cl,,,A], (c) (VAE [LX L]“)[(cl,,,A) nA is 2-uncountable+ (Vn)[{(x,, x,): a E S:}c cl,x,A)ll, (d) ~_{S~:~EW~AE[L~L]~}~~{T~:AE[L~L]”}=~,\{O}. Forp<o,letLp={x,. a < p} and let pB be the subspace topology on L inherited from p. Let T = {x, E L: (3A)[a E TJ}. For n < w, define L”={x,E L: (3msn)(3A)[aES,“]}u Tu{x,}. Observe that Un_, L” = L. A. BeSlagiC / Another Dowkerproduct 255 Using CH, list [Lx L]” as (AD : p E A) so that for each /?, A, c Lp x Lo. By induction on (YE A u {0}, we construct, for i -C 2 and (Ys p < (Y+ w, families { &Ji: k < w} of subsets of Lo+,. Let T; (i < 2) be the topology on L, generated by { LJb,k : k < w A p < -y} as a (sub)basis. We require, at stage (YE A u (0): (1) rL+, is a Hausdorff topology on L,,,, and rh+, = pa+, ; (2) for every p < (Y+ w, { lJb,k: k < w} is a clopen basis for xP in (La+,, 7nfw); (3) for every p < (Y+ o, ( I!JL,~: k < w) is decreasmg; (4) for every p < (Y+w, Lp+2 is clopen in (La+,, 7:+,); (5) for every p < (Y+ 0, rb = rh+, n P( Lp); (6) for every p < a! + o and n < w, if xp E L”, then L&n UL,oc L”; (7) for every p < (Y+ o, if x0 E T or p = 0, L&n Ui,, = {x0}. Fix an increasing sequence (LYE: j < w) of ordinals less than (Y, which converges to (Y. We require furthermore: (8) If a E TA, for some A, then for every sequence (p,: j < w) of ordinals less than (Y such that for each j < w, (i) A,, = (La\&) x (L,\L,), and (ii) (x,, X,+I)E clpxp& I we have that (9) if (YE Sl, for some A and n, then for every sequence (pj: j < w) of ordinals less than (Y such that for each j < w, (i) Ap,=(L,\L,,)x(L,\L,,), (ii) A, n A c L” x L”, and (iii) (x,, .G> E cl,&, we have that (10) if (YE Si, for some A and n, then for every sequence (p,: j < w) of ordinals less than CCsuch that for each j < w, (i) n(A,,)c L,\&,, and (ii) x, E cl,II(A,J we have that xc?E CL;,, U n(A,,) J<W (Recall that 17 is the projection map onto the first factor.) At the end we let ri be the topology on L generated by lJ {rb: (Y< w,} as a (sub)basis and let Xi = (L, d) for i ~2. We use (10) to show that both X0 and X, 256 A. BeflagiC / Another Dowkerproduct are normal and countably paracompact. Conditions (8) and (9) are used to show that X,xX, is normal. And (6), (7), and (9) show that the diagonal A with the subspace topology inherited from X,xX, is not countably paracompact, thus showing X,x X, Dowker. And then since the theorem mentions X and Y, we let X = X,, and Y = X1. (We did not want to have subscripts in the statement of the theorem.) Now we construct the topologies. If (Y= 0, the points Xj, j < w, are isolated. Set Uj,k = {Xj} for each i, j, k. Trivially, (l)-( 10) hold. Assume that (Y E TA for some A. Then points x0 for (Y+ 2 < /3 < (Y+ w are isolated, so set Ug, = {xP} for each i, k, and p with LY+ 2 s p < (Y+ w. In order to get (7) for x, and x,+1 we set, for k < w, Fix an increasing sequence (cyi: j < w) of ordinals (less than (u) converging to (Y. If {A,: /3 <a A (x,, x,+,)E cl,,,A,} is finite, set, for k< w, U&=(x,} and U’ol+l,k = {x,+~}. Otherwise list {A,: p -C a A (x,, xa+J E cl,,,A,} as (A’: 1<w}. For I< w pick a point d, E A’ whose distance in the Euclidean metric from (x,, x,+J is less than l/2’, and each coordinate of dl is different from any coordinate of any d,,,, m < 1. This can be done since each A’ c L, x L, and x,, x,,, r~ L,. Let d, = (dy, d:) for 1~ w. By the induction hypothesis, each T, is stronger than pa so the sets { dy : I< w} and {d: : I< 6.1)are closed discrete in 7: and 7: respectively. For I< w and i < 2 pick a clopen neighborhood Uf of df in 7: so that (i) the family { Ui : l< w} is discrete in T: ; (ii) the diameter of Uf in the Euclidean metric is les than l/2’; and (iii) for any j<,, if dfE LLI\La,+z,then Ufc L,\L,,.